This invention relates to a microwave heating material and a method of making the same. More particularly, this invention relates to a new and novel microwave heating or packaging material comprised of a dielectric substrate having a predetermined metallized pattern selectively disposed thereon for use in permanent and disposable food containers, food wrappers and certain industrial applications involving microwave heating. This invention also relates to a novel method of manufacturing a microwave packaging material utilizing a transfer process and preferably a hot stamping transfer process.
The following discussion of the prior art and the present invention will be in terms of its application in conjunction with containers and packaging for use with microwave ovens. It will be appreciated however, that the microwave packaging material and method of making the same is equally applicable to any other field wherein high intensity microwave heating is utilized.
It will be appreciated that the use of microwave energy (e.g. microwave ovens) in food preparation and cooling is now quite common. One of the primary problems with microwave energy cooking is that it fails to provide the proper so-called browning, searing and/or crisping to the foods which are normally expected to have such a quality. Therefore, foods of this type, after being heated or cooked in the microwave, do not possess the degree of appeal and taste normally expected.
Many attempts have been made to correct this lack of browning when microwave energy is used. One attempt to solve the browning problem is the inclusion of various modifications such as an electrical broiler element in the microwave oven to produce the short-wave energy needed to brown food. Another solution has been to use various types of eatable coatings, which only add the appearance of surface browning (such materials are designed to brown the food for aesthetics only).
Other attempts at solving the browning problem have been directed at modifications of the microwave energy itself by using special utensils, pans and packages to convert the high frequency microwave energy by resistance losses into heat. Heat gives the requisite browning effect and coloration to the food.
Some of these special utensils, pans and packages are illustrated in U.S. Pat. Nos. 3,302,632; 3,701,872; 3,777,099; 4,190,757; 4,230,924, 4,267,420, 4,268,738 and 4,434,197. Of particular interest with respect to the present invention are, the special packages illustrated in U.S. Pat. Nos. 4,230,924 and 4,267,420.
U.S. Pat. No. 4,230,924 to Brastad et al discloses a flexible dielectric substrate in the form of a sheet of plastic or paper having a metallic coating thereon. The metallic coating is sub-divided into a number of metallic islands or pads with non-metallic gaps or strips therebetween. The size of the gaps can be varied and, thus, permit selected amounts of microwave energy to be transmitted through the sheet. In related U.S. Pat. No. 4,267,420 to Brastad, a similar construction is disclosed except that the metallic coating is continuous and uninterrupted rather than being sub-divided into a plurality of islands or pads.
As is clear from a reading of the above discussed patents, particularly patent Nos. 4,230,924 and 4,267,420, the use of metallized (typically aluminized) films for microwave cooking has been well established. These films are usually comprised of laminates which contain one layer of controlled density metallized film. The metallized film consists of polyester, vacuum metallized to a level of 300 OHMS/Sq. (O.D. 0.18-0.21). This laminate is used to either wrap the food material; or is made rigid by a further lamination to boardstock and then cut and placed into a package which contains the food to be microwaved (this laminate can consist of only one film metallized and laminated to board).
Several problems and deficiencies are present in these prior art methods of manufacturing microwaveable metallized film laminates. For example, the inventors herein have found it extremely desirable to form the metallized layer in a preselected pattern disposed on a substrate. However, it is very difficult to create this high intensity metallized heating layer (leading to browning) in a pattern. Moreover, it is very expensive and labor intensive to cut a desired pattern (i.e., spirals, circles, dots, strips, etc.) from a metallic film and then bond and support these cut patterns on a substrate. In the case of board laminates, die cutting would be necessary and expensive board would be needed so that odors, charring and singeing would be avoided.